Negative Emissions Technologies (NETs) are crucial to avert catastrophic disruption of global climate patterns caused by the continuing atmospheric accumulation of CO2 due to industrial emissions. The recent surge of interest in NETs in which CO2 (currently at an atmospheric concentration of ~420 ppm) is removed from the ambient environment itself, by, e.g., direct air capture (DAC), has not yet been matched by a similar drive to reduce CO2 in oceans, where increasing acidification has led to destruction of coral reefs, and reduced carbonate ion concentrations harm shellfish and other marine life. The total CO2 accumulation rates by oceans rivals that in the atmosphere, and thus effective means for CO2removal could augment the other NETs to reduce the environmental burden imposed by this greenhouse gas. The concentrations in water (on a volumetric basis) are much higher at 100 mg/L than that in the ambient air (0.77 mg/L), and thus smaller volumes will need to be treated than in DAC, which could provide a processing advantage. Reported successful approaches to CO2 removal from oceanwaters rely on water splitting via, bipolar membrane catalysis and electrodialysis for pH modulation to release the CO2 as a gas (low pH) or carbonate salt (high pH). We describe an alternative, membrane-free, electro-swing system based on electrochemical modulation of the pH via reversible, chloride-mediated electrochemical reactions at the electrodes of electrochemical cells through which the ocean water is passed. This approach does not require expensive membranes or addition of chemicals, is easy to deploy, and does not lead to formation of byproducts or secondary streams. We discuss the overall process, with a focus on flexible electrode configurations to reduce transport and electrical resistances while enabling treatment of large quantities of water. Effective means for the recovery of molecular CO2 without the need for high vacuum desorption, and of calcium carbonate precipitates without fouling of the electrodes, are identified. We discuss the preparation and characterization of the electrodes, and report experimental results validating our approach for the electrochemical control of both CO2 release and CaCO3 recovery, bolstered by thermodynamic and transport modeling. A detailed techno-economic analysis of the overall process points to its viability to address growing climate concerns.

Asymmetric chloride-mediated electrochemical processes for CO2 removal from oceanwater

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